This dataset pertains to a cross-sectional investigation encompassing 129 individuals who have undergone weight-loss surgery. It features pre- and post-surgical measurements of weight and body mass index (BMI), alongside assessments of dietary restraint and dichotomous thinking. Analysis of the data substantiates the assertion that dichotomous thinking concerning food and diet acts as a mediator in the association between dietary restraint and the maintenance of weight loss subsequent to surgery. This finding holds clinical significance due to the measurable and modifiable nature of dichotomous thinking through psychological intervention. For further information, please refer to Marshall, Reay, & Bowman's (2024) work titled "Weight Loss After Weight-Loss Surgery: The Mediating Role of Dichotomous Thinking," published in Obesity Surgery. Note that the dataset in question contains no personally identifiable information, and participants provided informed consent for the dissemination of their de-identified research data in support of open science initiatives.